Since you're using Microsoft's parser, I'd suggest you contact one of
Microsoft's support channels to find out what they can or can't do for you.
The DOM doesn't yet have an official answer for this, thought DOM Level 3's
Load/Save chapter is expected to address it.
(The closest thing to a general solution right now is to use a SAX parser,
run the SAX stream through a filter that selects only those portions of the
document you're interested in, and run the resulting filtered SAX stream
into a SAX-to-DOM builder.)
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Joe Kesselman / IBM Research